Conventionally, as such an optical disk device, the one disclosed in JP2000-132848 A has been known, for example. Based on this precedent with a part thereof being modified, the following description is made with reference to FIGS. 26 to 30.
FIG. 26 is a schematic cross-sectional view showing a conventional optical disk device (in the case where a focal point of an objective lens is on a first signal plane of an optical disk). FIG. 27 shows a hologram pattern formed on a hologram that is used in the optical disk device. FIG. 28 shows a photodetection pattern formed on a photodetector that is used in the optical disk device and light distribution on the photodetector in the state shown in FIG. 26. FIG. 29 is a schematic cross-sectional view showing the conventional optical disk device (in the case where a focal point of the objective lens is on a second signal plane of the optical disk). FIG. 30 shows a photodetection pattern formed on the photodetector that is used in the optical disk device and light distribution on the photodetector in the state shown in FIG. 29.
As shown in FIG. 26, the conventional optical disk device includes a light source 1, a collimator lens 3 for converting light emitted from the light source 1 into parallel light, an objective lens 5 for focusing the parallel light on an optical disk, a hologram 4 for diffracting the light reflected by the optical disk (i.e., returned light), a beam splitter 2 for bending a light path of the returned light that has passed through the hologram 4 and then the collimator lens 3, and a photodetector 7 on which the returned light whose light path has been bent by the beam splitter 2 is focused.
The optical disk includes a substrate 6 made of a transparent material, a semi-transparent first signal plane 6a formed on a surface of the substrate 6, and a second signal plane 6b formed in proximity to the first signal plane 6a on the back side of the first signal plane 6a (i.e., on the side farther from the objective lens 5). The distance d between the first signal plane 6a and the second signal plane 6b generally is about 20 to several tens μm, and a transparent medium having a refractive index n(n=about 1.5) fills a space between the first signal plane 6a and the second signal plane 6b. 
As shown in FIG. 27, the hologram 4 is divided into four equal parts, namely, a first quadrant 41, a second quadrant 42, a third quadrant 43, and a fourth quadrant 44, by straight lines that intersect with each other at an intersection point 40 of the optical axis and a surface of the hologram 4. A pattern is formed in each of these four regions.
As shown in FIG. 28, the photodetector 7 includes detector cells 7F1, 7F2, 7F3, and 7F4 divided by straight lines 7Fa and 7Fb and detector cells 7T1, 7T2, 7T3, and 7T4 divided by straight lines 7Ta, 7Tb, and 7Tc.
As shown in FIG. 26, light emitted from the light source 1 passes through the beam splitter 2 and is converged by the collimator lens 3 to turn into parallel light. After passing through the hologram 4, the parallel light is focused on the first signal plane 6a of the optical disk by the objective lens 5 (the light path is indicated with a solid line). Returned light 8a, which is the light reflected by the first signal plane 6a, is converged by the objective lens 5 and enters the hologram 4 to be diffracted by the hologram 4. The returned light 8a diffracted by the hologram 4 then is converged by the collimator lens 3 and is reflected by a split plane 2a of the beam splitter 2, whereby the light path of the returned light 8a is bent. The returned light 8a whose light path has been bent then is focused on the photodetector 7 (the light path of a 0th-order diffracted light beam resulting from the diffraction by the hologram 4 is indicated with a solid line).
The first signal plane 6a of the optical disk is semi-transparent. Thus, among light focused on the first signal plane 6a, light passing through the first signal plane 6a reaches the second signal plane 6b. Then, returned light 8b, which is the light reflected by the second signal plane 6b, passes through the first signal plane 6a again, is converged by the objective lens 5, and enters the hologram 4 to be diffracted by the hologram 4. The returned light 8b diffracted by the hologram 4 then is converged by the collimator lens 3 and is reflected by the split plane 2a of the beam splitter 2, whereby the light path of the returned light 8b is bent. The returned light 8b whose light path has been bent then is focused on the photodetector 7 (the light path of a 0th-order diffracted light beam is indicated with a dashed line). Note here that the focal point of the returned light 8b is on the front side of the focal point of the returned light 8a (i.e., on the side closer to the beam splitter 2).
Light reflected by the optical disk, i.e., returned light 8, is divided equally (or substantially equally) into four light beams, namely, a first quadrant light beam 81a (or 81b), a second quadrant light beam 82a (or 82b), a third quadrant light beam 83a (or 83b), and a fourth quadrant light beam 84a (or 84b) by the first quadrant 41 to the fourth quadrant 44, respectively, where the first quadrant light beam 81a to the fourth quadrant light beam 84a refer to the light beams derived from the returned light 8a that is reflected by a signal plane on/from which signals are recorded/reproduced (in this case, the first signal plane 6a) and enters the first quadrant 41 to the fourth quadrant 44 of the hologram 4, respectively, and the first quadrant light beam 81b to the fourth quadrant light beam 84b refer to the light beams derived from the returned light 8b that is reflected by a signal plane on/from which signals are not recorded/reproduced (in this case, the second signal plane 6b) and enters the first quadrant 41 to the fourth quadrant 44 of the hologram 4, respectively. These quadrant light beams are diffracted in the respective quadrants.
When the returned light 8a is projected on the photodetector 7, a 1st-order diffracted light beam 8a1 derived from the first quadrant light beam 81a forms a light spot 8a1S astride the border between the detector cells 7F1 and 7F4, a −1st-order diffracted light beam 8a1′ derived from the first quadrant light beam 81a forms a light spot 8a1S′ within the detector cell 7T1, a 1st-order diffracted light beam 8a2 derived from the second quadrant light beam 82a forms a light spot 8a2S astride the border between the detector cells 7F1 and 7F4, a −1st-order diffracted light beam 8a2′ derived from the second quadrant light beam 82a forms a light spot 8a2S′ within the detector cell 7T2, a 1st-order diffracted light beam 8a 3 derived from the third quadrant light beam 83a forms a light spot 8a3S astride the border between the detector cells 7F2 and 7F3, a −1st-order diffracted light beam 8a3′ derived from the third quadrant light beam 83a forms a light spot 8a3S′ within the detector cell 7T3, a 1st-order diffracted light beam 8a4 derived from the fourth quadrant light beam 84a forms a light spot 8a4S astride the border between the detector cells 7F2 and 7F3, and a −1st-order diffracted light beam 8a4′ derived from the fourth quadrant light beam 84a forms a light spot 8a4S′ within the detector cell 7T4 (the respective light spots are indicated with solid lines).
When the returned light 8b is projected on the photodetector 7, a 1st-order diffracted light beam 8b1 derived from the first quadrant light beam 81b forms a light spot 8b1S, a −1st-order diffracted light beam 8b1′ derived from the first quadrant light beam 81b forms a light spot 8b1S′, a 1st-order diffracted light beam 8b2 derived from the second quadrant light beam 82b forms a light spot 8b2S, a −1st-order diffracted light beam 8b2′ derived from the second quadrant light beam 82b forms a light spot 8b2S′, a 1st-order diffracted light beam 8b3 derived from the third quadrant light beam 83b forms a light spot 8b3S, a −1st-order diffracted light beam 8b3′ derived from the third quadrant light beam 83b forms a light spot 8b3S′, a 1st-order diffracted light beam 8b4 derived from the fourth quadrant light beam 84b forms a light spot 8b4S, and a −1st-order diffracted light beam 8b4′ derived from the fourth quadrant light beam 84b forms a light spot 8b4S′ (the respective light spots are indicated with dashed lines).
A point 80S shown in FIG. 28 is a focal point of 0th-order diffracted light beams derived from the first quadrant light beam 81a, the second quadrant light beam 82a, the third quadrant light beam 83a, and the fourth quadrant light beam 84a passing through the respective quadrants of the hologram 4 on the photodetector 7, and the light spots 8a1S, 8a2S, 8a3S, and 8a4S and the light spots 8a1S′, 8a2S′, 8a3S′, and 8a4S′ are minutely converged light spots that are close to diffraction focal points.
Therefore, it is possible to set the width w of the photodetector 7 to be as small as about 60 μm in accordance with the size of these light spots. Furthermore, since the focal point of the returned light 8b is on the front side of the focal point of the returned light 8a (i.e., on the side closer to the beam splitter 2), the light spots 8b1S, 8b2S, 8b3S, and 8b4S and the light spots 8b1S′, 8b2S′, 8b3S′, and 8b4S′ have shapes similar to those obtained by inverting the first quadrant light beam 81b, the second quadrant light beam 82b, the third quadrant light beam 83b, and the fourth quadrant light beam 84b with respect to the intersection point 40, respectively, and most of their regions are on the detector cells 7F2 and 7F4 and the detector cells 7T1, 7T2, 7T3, and 7T4 as stray light components.
FIG. 29 is the same as FIG. 26 except that the focal point of the objective lens 5 is on the second signal plane 6b of the optical disk (i.e., the second signal plane 6b is a signal plane on/from which signals are recorded/reproduced) and that the focal point of the returned light 8b reflected by the first signal plane 6a is on the back side of the focal point of the returned light 8a reflected by the second signal plane 6b (i.e., on the side farther from the beam splitter 2). The duplicate description will be omitted here.
Similar to FIG. 28, a point 80S shown in FIG. 30 is a focal point of 0th-order diffracted light beams derived from the first quadrant light beam 81a, the second quadrant light beam 82a, the third quadrant light beam 83a, and the fourth quadrant light beam 84a passing through the respective quadrants of the hologram 4 on the photodetector 7, and the light spots 8a1S, 8a2S, 8a3S, and 8a4S and the light spots 8a1S′, 8a2S′, 8a3S′, and 8a4S′ are minutely converged light spots that are close to diffraction focal points.
In the optical disk device shown in FIG. 29, the focal point of the returned light 8b is on the back side of the focal point of the returned light 8a, unlike the case of FIG. 26. Thus, the light spots 8b1S, 8b2S, 8b3S, and 8b4S and the light spots 8b1S′, 8b2S′, 8b3S′, and 8b4S′ have shapes similar to those of the first quadrant light beam 81b, the second quadrant light beam 82b, the third quadrant light beam 83b, and the fourth quadrant light beam 84b, respectively, and most of their regions are on the detector cells 7F1, 7F2, 7F3, and 7F4 and the detector cells 7T1, 7T2, 7T3, and 7T4 as stray light components.
Some of the detector cells are electrically connected, so that the following six signals can be obtained.                F1=a signal obtained in the detector cell 7F1+a signal obtained in the detector cell 7F3        F2=a signal obtained in the detector cell 7F2+a signal obtained in the detector cell 7F4        T1=a signal obtained in the detector cell 7T1        T2=a signal obtained in the detector cell 7T2        T3=a signal obtained in the detector cell 7T3        T4=a signal obtained in the detector cell 7T4        
With the arrow 6R shown in FIG. 27 indicating the radial direction of the optical disk, a focus error signal FE that indicates an error in focusing light on the optical disk signal plane, a tracking error signal TE that indicates an error in tracking an optical disk track, and a reproduction signal RF that is reproduced from the optical disk signal plane are detected based on the following formulae (1) to (3).FE=F1−F2  Formula (1)TE=T1+T4−T2−T3  Formula (2)RF=F1+F2+T1+T2+T3+T4  Formula (3)
FIG. 31 shows the relationship between defocus and a focus error signal FE in the conventional optical disk device. FIG. 31 shows the relationship in the case where the photodetector 7 has a width w of 60 μm and the optical disk has the first signal plane 6a alone as a signal plane and is not provided with the second signal plane 6b (alternatively, the optical disk has the second signal plane 6b alone as a signal plane and is not provided with the first signal plane 6a). Note here that the defocus caused when the objective lens 5 approaches the signal plane is regarded as the negative (−) defocus. An FS signal in FIG. 31 is represented by F1+F2. In the conventional optical disk device, a detector shape formed by the detector cells 7T1, 7T2, 7T3, and 7T4 as a whole substantially is equal to that formed by the detector cells 7F1, 7F2, 7F3, and 7F4 as a whole. Thus, it may be considered that the reproduction signal RF that is reproduced from the optical disk signal plane is detected based on the following formula (4).RF=2×FS  Formula (4)
The above-described conventional optical disk device has the following problem. In the conventional optical disk device, most of the regions of the light spots 8b1S, 8b2S, 8b3S, and 8b4S and the light spots 8b1S′, 8b2S′, 8b3S′, and 8b4S′ are on the detector cells 7F1, 7F2, 7F3, and 7F4 and the detector cells 7T1, 7T2, 7T3, and 7T4. In the conventional optical disk device, it is possible to reduce an area of the portions where the light spots overlap the detector cells by reducing the width w of the photodetector 7. However, considering the margin needed for a position error of the light spots relative to the photodetector 7, the smallest possible width w of the photodetector 7 is about 60 μm. In the case where the distance d between the first signal plane 6a and the second signal plane 6b is 25 μm and the refractive index n of the transparent medium filling the space between the first signal plane 6a and the second signal plane 6b is 1.57, the effect of the second signal plane 6b during a focusing operation with respect to the first signal plane 6a corresponds to the state where the defocus d/n=−16 μm in FIG. 31, and the effect of the first signal plane 6a during a focusing operation with respect to the second signal plane 6b corresponds to the state where the defocus d/n=16 μmin FIG. 31. Especially when the defocus d/n=16 μm, the focus error signal FE includes a non-negligible offset amount (an amplitude A in FIG. 31). Thus, accurate focusing with respect to a focus control plane is disturbed due to the effect of stray light from a plane located in proximity to a plane on which light is focused (such a plane hereinafter also referred to simply as “a proximity plane”, and in this case, the first signal plane 6a is the proximity plane). As a result, accurate signal reading or writing cannot be performed.
Furthermore, a light spot formed by light from the second signal plane 6b during the focusing operation with respect to the first signal plane 6a corresponds to a light spot obtained by defocusing a light spot formed by light from the first signal plane 6a by −16 μm, and a light spot formed by light from the first signal plane 6a during the focusing operation with respect to the second signal plane 6b corresponds to a light spot obtained by defocusing a light spot formed by light from the second signal plane 6b by 16 μm. Therefore, assuming that the same amount of reflected light is detected from the first signal plane 6a and the second signal plane 6b during the focus control, the ratio of an amount of stray light from the second signal plane 6b to an amount of signal light from the first signal plane 6a during the focusing operation with respect to the first signal plane 6a corresponds to A1/A0, and the ratio of an amount of stray light from the first signal plane 6a to an amount of signal light from the second signal plane 6b during the focusing operation with respect to the second signal plane 6b corresponds to A2/A0. According to FIG. 31, these values fall within the range from 17% to 24%. The amount of stray light varies depending on the recording state of the proximity plane or the presence of an address pit, which makes accurate reading of a reproduction signal from the focus control plane difficult.
Moreover, in the case where signals are recorded on the second signal plane 6b, an amount of light passing through the first signal plane 6a varies depending on the recording state of the first signal plane 6a or the presence of an address pit, which causes the intensity of a light spot for recoding singles on the second signal plane 6b to vary, thereby making accurate signal writing difficult.